Solar cells are generally made of semiconductor materials, such as silicon (Si), which convert sunlight into useful electrical energy. Solar cells are, in general, made of thin wafers of Si in which the required PN junction is formed by diffusing phosphorus (P) from a suitable phosphorus source into a P-type Si wafer. The side of the silicon wafer on which sunlight is incident is generally coated with an anti-reflective coating (ARC) to prevent reflective loss of sunlight, which increases the solar cell efficiency. A two dimensional electrode grid pattern known as a front contact makes a connection to the N-side of silicon, and a coating of aluminum (Al) makes connection to the P-side of the silicon (back contact). Further, contacts known as silver rear contacts, made out of silver or silver-aluminum paste are printed and fired on the N-side of silicon to enable soldering of tabs that electrically connect one cell to the next in a solar cell module. These contacts are the electrical outlets from the PN junction to the outside load.
Conventional pastes for solar cell contacts contain lead frits. Inclusion of PbO in a glass component of a solar cell paste has the desirable effects of (a) lowering the firing temperature of paste compositions, (b) facilitating interaction with the silicon substrate and, upon firing, helping to form low resistance contacts with silicon. For these and other reasons PbO is a significant component in many conventional solar cell paste compositions. However, in light of environmental concerns, the use of PbO (as well as CdO), in paste compositions is now largely avoided whenever possible. Hence a need exists in the photovoltaic industry for lead-free and cadmium-free paste compositions, which afford desirable properties using lead-free and cadmium-free glasses in solar cell contact pastes.